(1) Field of the Invention
This invention relates to a sintered aluminum-base alloy and method for producing the same. The sintered aluminum alloy of the present invention is characterized in strength, small weight and excellent wear resistance. Accordingly, it is suitable for use in producing parts of machinery such as gearwheels, pulleys, compressor vanes, connecting rods, and pistons, in which the excellence in the above properties are required.
(2) Description of Prior Art
In view of economy in energy consumption and improvement in mechanical efficiency, demands for lightweight machine parts are increased. Because it is possible for a sintered aluminum alloy that the content of fine crystals of pro-eutectic Si can be increased as compared with cast alloys, the sintered aluminum alloy is expected as a material having excellent specific strength and wear resistance.
As a conventional method for producing sintered aluminum alloy, Japanese Laid-open Publication No. 53-128512 discloses a method of mixing some members selected from the group consisting of Al-10/35% Si powder, Cu powder, Mg powder, Al-Cu powder, Cu-Mg powder, Al-Cu-Mg powder, Cu-Mg-Si powder, and Al-Cu-Mg-Si powder, and if necessary, further adding Al powder to obtain a composition consisting of, in weight-basis, 0.2-4% Cu, 0.2-2% Mg, 10-35% Si, and the balance of Al, then compacting the powder mixture and sintering the obtained green compact to produce a desired product. This method is the so-called mixing method in which several powders are mixed together. Because soft metal powder can be mixed in the method of this kind, the compacting process can be improved. Furthermore, because fairly strong sintered products can be produced only by the conventional compacting and sintering processes, this mixing method is employed for the production of various machine parts of which special strength is not required.
Besides the above-described method, a sintered product of rapidly solidified aluminum alloy is disclosed in Japanese Laid-open Patent Publication No. 62-10237, in which pro-eutectic Si crystals are uniformly dispersed in an Al-Si alloy matrix. This alloy has a composition, in terms of weight, of 10-30% Si, 1-15% in total of one or more members of Ni, Fe and Mn, and if necessary, 0.5-5% Cu and 0.2-3% Mg, and the balance of Al and unavoidable impurities, and the alloy product is prepared through compacting and hot press forging processes. According to this alloying method, highly strong products can be obtained as compared with those prepared by the mixing method. However, because the powder which is prepared by rapid solidification is hard, the near-net shaping using a metal mold is difficultly carried out, powder particles are coated with hard oxide films and any liquid phase is not produced during the sintering. Therefore, the sufficient combining of powder particles cannot be attained only by sintering and repeated pressing operations such as extrusion from billet forms and forging are required. Accordingly, there remain some problems in this method in view of workability and production cost.
In order to solve the above problems, another method is proposed in Japanese Laid-open Patent Publication No. 5-156399 as a combination of mixing method and alloying method. The alloy product is prepared by mixing a certain amount of pure Al powder with rapidly solidified Al-Si alloy powder and the powder mixture is subjected to hot press forging. Its composition in terms of weight is 12-30% Si, 1-10% of one or both of Fe and Ni, and if necessary, one or both of 1-5% Cu and 0.3-2% of Mg, and the balance of Al and unavoidable impurities. In the grain structure of this alloy, 5-20 vol.% of the grains of Al-solid solution which are deformed in hot forging process, are dispersed in an eutectic Al-Si alloy matrix containing dispersion of fine pro-eutectic Si crystals. In this alloy, the Al-solid solution acts as an adhesive to improve the mutual close adhesion among hard particle boundaries. As a result, the wear resistance and strength are improved.
Meanwhile, with the tendency to employ aluminum alloy parts for various high-performance machinery, those which have relatively high strength and is especially high in wear resistance, are demanded in the industry.
Although the above-described conventional alloys have their own advantages, the reason why the ductility of alloys made by mixing method is not so high enough, is considered that, when liquid phase sintering is carried out in the state in which pro-eutectic Si crystals do not become coarse, the Cu which is added to improve the strength of alloy matrix cannot be dispersed sufficiently in the matrix and it precipitates in the form of intermetallic compound in the vicinity of grain boundaries with reducing the ductility.
Furthermore, in the conventional Al-Si alloy, fine pro-eutectic Si crystals are uniformly dispersed, so that both the strength and wear resistance are high. However, in view of the state of wearing, the hard pro-eutectic Si crystals which are released from the surface of alloy matrix in sliding contact may act as an abrasive. Therefore, there is room for betterment in the conventional aluminum alloy.